package com.star.common.utils;

public class SnowflakeIdUtil {

    /**开始时间戳 (2015-01-01)**/
    private final long twepoch = 1420041600000L;

    /**机器id所占的位数**/
    private final long workerIdBits = 5L;

    /**数据标识id所占的位数**/
    private final long datacenterIdBits = 5L;

    /**支持的最大机器id，结果是31(这个移位算法可以很快的计算出几位二进制数所能表示的最大)**/
    private final long maxWorkerId = -1L ^ (-1L<< workerIdBits);

    /**支持的最大数据表示id，结果是31**/
    private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);

    /**序列在id中占的位数**/
    private final long sequenceBits = 12L;

    /**机器ID向左移12位**/
    private final long workerIdShift = sequenceBits;

    /**数据标识id向左移17位(12+5)**/
    private final long datacenterIdShift = sequenceBits + workerIdBits;

    /**时间戳向左移22位(5+5+12)**/
    private final long timestampLeftShift = sequenceBits + workerIdBits +datacenterIdBits;

    private final long sequenceMask = -1L ^ (-1L << sequenceBits);

    /** 工作机器ID(0 ~ 31) **/
    private long workerId;

    /** 数据中心ID(0~31) **/
    private long datacenterId;

    /**毫秒内序列(0~4095)**/
    private long sequence = 0L;

    /**上次生成ID的时间戳**/
    private long lastTimestamp = -1L;

    /**
     * 构造函数
     * @param workerId 工作ID
     * @param datacenterId 数据中心ID
     */
    public SnowflakeIdUtil(long workerId, long datacenterId) {
        if (workerId > maxDatacenterId || workerId < 0){
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0",maxWorkerId));
        }
        if(datacenterId > maxDatacenterId || datacenterId < 0){
            throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0",maxDatacenterId));
        }
        this.workerId = workerId;
        this.datacenterId = datacenterId;
    }

    public synchronized long nextId(){
        long timestamp = timeGen();
        //如果当前时间小于上一次ID生成的时间戳，说明系统时钟回退过 这个时候应当抛出异常
        if(timestamp < lastTimestamp){
            throw new RuntimeException(
                    String.format("Clock moved backwards. Refusing to generate id for %d milliseconds",lastTimestamp-timestamp));
        }

        //如果是同一时间生成的，则进行毫秒内序列
        if(lastTimestamp == timestamp){
            sequence = (sequence  + 1) & sequenceMask;
            //毫秒内序列溢出
            if(sequence == 0){
                //阻塞到下一个毫秒，获得新的时间戳
                timestamp = tilNextMillis(lastTimestamp);
            }
        }else{
            //时间戳改变，毫秒内序列重置
            sequence = 0L;
        }
        //上次生成ID的时间戳
        lastTimestamp = timestamp;

        //移位并通过运算拼到一起组成64位的ID
        return ((timestamp - twepoch) << timestampLeftShift)
                | (datacenterId << datacenterIdShift)
                | (workerId << workerIdShift)
                | sequence;
    }

    /**
     * 阻塞到下一个毫秒，直到获得新的时间戳
     * @param lastTimestamp 上次生成ID的时间戳
     * @return 当前时间戳
     */
    protected long tilNextMillis(long lastTimestamp){
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp){
            timestamp = timeGen();
        }
        return timestamp;
    }

    /**
     * 返回以毫秒为单位的当前时间
     * @return 当前时间(毫秒)
     */
    protected long timeGen(){
        return System.currentTimeMillis();
    }

    public static void main(String[] args) {
        SnowflakeIdUtil idWorker = new SnowflakeIdUtil(3,1);
        System.out.println(idWorker.nextId());
    }

}
